1. Field of the Invention
This invention relates to improvements in coating apparatus. The invention is particularly useful in the coating of one or more layers of coating compositions onto a substrate, such as, say, a web.
2. Description of the Prior Art
In the prior art, as represented by U.S. Pat. Nos. 2,761,419, filed in the name of Mercier et al. and 2,761,791 filed in the name of Russell et al.--the contents of which patents are herein incorporated by reference--it is known to simultaneously coat a moving substrate, for example, a web of photographic film base with emulsion coating compositions, by the use of a coating hopper which includes one or more inclined slide surfaces, down which one or more of the fluid coating compositions may be made to flow. The hopper may include a plurality of separate exit slots, whereby respective liquid coating compositions may be metered from individual supplies and distributed uniformly across respective inclined slide surfaces. Each of the respective compositions flows by gravity as a layer down its respective inclined surface, whereby the layer becomes smooth and of uniform thickness. The slide surfaces are arranged so that the layers flow on top of one another. At the end of the last slide surface, i.e., the one adjacent to the substrate, the stratified layers flow into a bead or puddle which bridges a small generally horizontal gap between an edge of the coating hopper and the upwardly moving substrate. The substrate, as it is advanced into contact with the bead, simultaneously picks up all the layers, which layers deposit on the substrate as a composite coating of substantially distinct superimposed layers.
While the above apparatus functions satisfactorily, various defects can arise in the coated layers if the bead is disturbed. For example, a particularly noticeable fault is the appearance of longitudinal striations which render the coated web unacceptable as a commercial photographic product. The presence of these longitudinal striations tends to increase rapidly as the web speed is increased.
Furthermore, as disclosed in the aforementioned patent, and in U.S. Pat. No. 2,681,294--the contents of which are also incorporated by reference--it is advantageous to create a pressure differential between the exposed surfaces of the bead, such as by the creation of a vacuum on the trailing surface of the bead to eliminate excessive vibration and/or rupture of the bead. The range of vacuum levels which may be used is, of course, limited as, if it is too great, the high pressure differential across the bead may also cause the bead to be disturbed and/or ruptured. At times, it may develop that the amount of vacuum being used is insufficient to elimiate the excessive vibration in the bead, and thus it is desirable to increase the amount of such vacuum. Where an increase in vacuum will itself cause disturbance and/or rupture of the bead, it is sometimes necessary to change coating conditions, such as by lowering web speed, to make satisfactory coatings.
A further problem with the use of the coating hopper described in the aforementioned Mercier et al patent is that particles in the liquid coating composition may adhere to the hopper at the lip edge thereof and cause undesirable streaks to be formed in the coating. An apparatus which reduces or minimizes the deleterious effects of such particles on coatings represents a significant contribution to the state of the art.